Gallery, Projects and General > Project Logs |
Flames and flame suckers and eaters |
<< < (2/4) > >> |
John Hill:
--- Quote from: bogstandard on March 24, 2011, 12:52:55 PM ---Why do they go faster when reaching a higher temperature, in fact some just won't run until a critical temp is reached? --- End quote --- That does make me :scratch: to come up with a possible explanation and the best I can manage is to propose that the cold cylinder cools the gases too fast and the equal pressure point ocurrs long before the valve opens and the high pressure at that point stalls the action. A fairly weak theory though.. Makes me :coffee: though, if the engine had a passive exhaust valve (I think some people put a little ball valve on top of the cylinder?) that might make a cold engine easier to start and would also avoid the puff that blows the flame out..... but it would have to be a very sensitive valve. :scratch: |
Bogstandard:
Gents, I wasn't trying to come up with the theory of how its works. I was just raising a few points that required thinking about, not this is, but what ifs. I am absolutely positive that somewhere, it isn't just the cooling down of the hot gases that causes them to work as they do. The reason I suspect it is to do with unburnt hydrocarbons was when I was doing trials with both gas and spirit burners, the engine didn't like working with the hottest parts of the flame, which is what you would expect, but areas where unburnt fuels would be expected to be present. John |
NickG:
Hi guys, sorry if this is a bit long winded but here are my thoughts. I see the first theory John talked about slightly differently and it took me a while to get around to this way of thinking. I'm not sure if it's correct, but it makes more sense to me and it links a bit more to the laws of thermodynamics from what little I can remeber. The hot gases, as you say are drawn into the cylinder - and the valve is closed. So you have in there very hot gases - and this is the important bit - at atmospheric pressure, occupying a certain volume. When the gases hit the relatively cold cylinder the gas cools and hence the pressure in the cylinder is greatly reduced to below atmospheric pressure. Hence there is an over pressure on the other side of the piston (atmospheric pressure). I believe this is why they are sometimes referred to as atmospheric engines. One theory about why they work better once up to a certain temperature is what I've heard called the quenching effect. That is, when the engine is cold the hot gasses cool so quickly the under-pressure is created before the valve is shut, so this just causes an influx of air into the cylinder and it ends up back nearer to atmospheric pressure so there is little or no differential. I concur with Chuck and John Hill here, the main reason for the pressure that blows out when the piston is nearing the top of its stroke, I think, is as follows. Imagine the hot gases have been sucked in, the piston is now at bottom dead centre and is just about to start its return journey. So the gases occupy the swept volume of the cylinder at atmospheric pressure, the valve is shut and the gases have rapidly cooled. There is now a pressure less than atmospheric (Note: it may not be much less than atmospheric, which is why these engines have so little power. If you think about it, if there was a perfect vacuum, the pressure difference would be 1 atmosphere 14.7 psi - which would actually give you quite a lot of power with a 3/4" or so Bore!) so the atmosphere on the other side of the piston starts to push the piston back towards top dead centre. Now the volume inside the cylinder is decreasing rapidly so the pressure is actually rising back towards atmospheric. Once it goes above atmospheric on most flame lickers the valve will be forced open as there is now an over pressure INSIDE the cylinder. That's what the popping noise is and the pressure blowing out over the flame. This is very obvious on 'poppin' and mad jack's engine and it's easy for the engine to do with the reed type valve. I guess an efficient and well designed engine would have the pressure inside and out not equalise until the piston was right near the top of it's stroke. That would take some serious maths to work out though! A lot of engines actually have a secondary spring loaded relief or exhaust valve incorporated into the design to relive that pressure, otherwise with a rigidly held valve the engine would be fighting against that pressure. Incidentally, on Jan Ridders internal valve engine it has a spigot on the piston that pushes the valve open, but in actual fact, when the pressures equalise the pressure forces the valve open the first little bit until the port is cracked open then the spigot makes sure it's fully open ready to draw in the next flame. What John said about the incomplete combustion I'd never thought about, but I must admit, even before I'd seen Chucks reply I thought exactly the same as him. If combustion was continuing, I think the pressure would be more likely to increase rather than decrease. They are just my thoughts and from what I've read / learned over the years. As John (Bogs) said though, neither theory fully supports the some of the behaviours we see in these engines. My poppin will start almost instantly but then stop, then it will need to warm up for a few mins to run reliably! (is that just until it gets rid of moisture build up though?) Also, another strange one as Bogs mentioned, the poppin seems to prefer sucking just a bit of the flame in like Jack's seems to - but how could that make it run faster? I think like you, there must be more to it, but if it was hydrocarbons surely that would produce a higher pressure, not use what little oxygen is in there and help produce less pressure? Think it must be bloomin' complex! By the way madjack, the engine seems to be running superbly now and I think you've optimised the valve set up by moving it upwards leaving the port unobstructed and having the cylinder at a good angle. :thumbup: Nick |
madjackghengis:
Hi all, glad to have all the different perspectives added to mine. In watching them run, I note they all sweat moisture immediately, and postulate since the basic exhaust of an alcohol burner is water vapor and carbon dioxide, the moisture immediately mixes with any oil residue making a sticky emulsion. When I spin my cold engine over, it spins freely, but four or five cycles of burner input has it sticking, and I can stick a cotton swab in the port and pull out yellow emulsion. When I wash it out with WD, I immediately get high speed, the feeling of the solvent burning in addition to the alcohol, adding to the expansion during the valve open, and it generally gets hot enough no water condenses on either the valve or the head and cylinder, leaving just the oil residue. I've seen the additional valve in some, and have seen a ball sitting by its weight to allow the cooled air to exhaust without going through the port, so I may try this with the next one I'm in the middle of, as there is definitely a volume of air left in the cylinder when it has cooled, causing the pop and blowing out flame. The alcohol and the butane are the fuels of choice because they leave so little partially burned hydrocarbons, as well as burning at a lower temperature. These engines need a volume of hot air, hot enough for the temperature differential to allow substantial volume change, but the volume of the hot air must essentially fill the cylinder by the time the port closes, and having a hotter flame with less volume does not accomplish this, as tested with several small butane torches, requiring a "soft flame", as John has said, to provide the volume of hot gasses. I would also note, what Nick said about the cam closing the valve, and allowing the spring to open it when the vacuum allows, has been a major factor in the better performance of the oddball, which has been my test piece. I also note that when I take my Duclos flame sucker, and set it up, brass cylinder, bronze piston, bronze valve, with the tip of the flame just a quarter of an inch from the port when cold, and give it a good spin, if I keep that red, meaning fully burned, no unburned fuel left, as in the blue part, so it is almost exclusively pulled into the port, it will come up to about half speed immediately and if I carefully maintain that tip just going in, while the rest of the flame goes across the valve, pulling it further away as it speeds up, I get no condensation, no popping, and it goes from half speed up to full speed in about thirty seconds of running, and will continue at that speed until the graphite pencil rubbings are gone and it sticks, some fifteen minutes or so. The Duclos works well this way, but the oddball is arranged such that it doesn't have as much flame for its volume, and I can't get just the top of the flame going in, as I do with the Duclos. I believe the designer of the "poppin" was right in his putting a thin edge on the port, to minimize temperature change there, as my head is almost a quarter inch thick, and takes some time before it quits sweating moisture. If I fail with the Duclos, and get too much flame in say twice, it will require being spun over until warm before it will start off, and will pop, showing the valve is sticking due to the moisture wicking between it and the cylinder. I would suggest that is where your engine is popping or quacking, John, as the cooled air is forced out against valve pressure, causing the valve to act as a reed, even as a block of graphite. I suspect engines that are closer to "square" are more efficient as well, as long strokes mean lots of contact area between pistons and cylinders. I've reached the point with the oddball, where the valve timing is controlling the speed, and its opening is controlled by the vacuum holding the reed until the pressure equalizes, and it is almost silent once it is hot and running at about six hundred rpm. I appreciate all the chiming in, lots of good ideas to consider. Cheers, Jack |
BillTodd:
Another take on it.... The flame that is sucked into the cylinder is NOT a gas, it is a plasma, a different state of matter, as its temperature drops it condenses rapidly to a gas dramatically reducing in volume . All gases have a poor conductivity so contraction due to cooling is fairly slow - the plasma loses heat by radiation (those ultra violet/blue photons carry a lot of energy away) so it condenses very rapidly. I suspect the reason why a cold engine is difficult to start is because the plasma condenses too quickly - while the valve is still open - thus not producing the lower pressure required. Bill |
Navigation |
Message Index |
Next page |
Previous page |